Electronic control of the spin–orbit branching ratio in the photodissociation and predissociation of HCl (original) (raw)
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Photodissociation of HCl at 193.3 nm: Spin–orbit branching ratio
The Journal of Chemical Physics, 1997
HCl was photodissociated by ultraviolet ͑uv͒ radiation at 193.3 nm. Time-of-flight spectra of the hydrogen atom fragment provided the spin-orbit state distribution of the chlorine fragment, ͓Cl(2 P 1/2)͔/͓Cl(2 P 3/2)͔ϭ0.69Ϯ0.02, in excellent agreement with recent theoretical studies. The H atom angular distribution studied by changing the uv photolysis laser polarization confirmed a dominant A 1 ⌸←X 1 ⌺ ϩ electronic transition in the photoexcitation process ͑ϭϪ1.01Ϯ0.04 and *ϭϪ0.94Ϯ0.07͒.
Recoil velocity-dependent spin–orbit state distribution of chlorine photofragments
Chemical Physics, 2004
We present the results of experimental and theoretical studies of the speed-dependent spin-orbit state distributions of chlorine photofragments produced in the photodissociation of thiophosgene (CSCl 2 ) at 235 nm. Three-dimensional imaging has been employed for observing chlorine photofragments in their ground (Cl) and excited (Cl à ) spin-orbit states. The kinetic energy distributions for Cl and Cl à fragments reflect excitation of several electronic states of the partner fragment CSCl. The spin-orbit branching ratio of P(Cl à )/[P(Cl) + P(Cl à )] was found to depend on the kinetic recoil energy increasing from 0.1 for low kinetic energy to 0.8 for high kinetic energy.
The Journal of chemical physics, 2004
The photodissociation dynamics of allyl chloride at 235 nm producing atomic Cl( 2 P J ;J ϭ1/2,3/2) fragments is investigated using a two-dimensional photofragment velocity ion imaging technique. Detection of the Cl( 2 P 1/2 ) and Cl( 2 P 3/2 ) products by ͓2ϩ1͔ resonance enhanced multiphoton ionization shows that primary C-Cl bond fission of allyl chloride generates 66.8% Cl( 2 P 3/2 ) and 33.2% Cl( 2 P 1/2 ). The Cl( 2 P 3/2 ) fragments evidenced a bimodal translational energy distribution with a relative weight of low kinetic energy Cl( 2 P 3/2 )/high kinetic energy Cl( 2 P 3/2 ) of 0.097/0.903. The minor dissociation channel for C-Cl bond fission, producing low kinetic energy chlorine atoms, formed only chlorine atoms in the Cl( 2 P 3/2 ) spin-orbit state. The dominant C-Cl bond fission channel, attributed to an electronic predissociation that results in high kinetic energy Cl atoms, produced both Cl( 2 P 1/2 ) and Cl( 2 P 3/2 ) atomic fragments. The relative branching for this dissociation channel is Cl( 2 P 1/2 )/͓Cl( 2 P 1/2 )ϩCl( 2 P 3/2 )͔ϭ35.5%. The average fraction of available energy imparted into product recoil for the high kinetic energy products was found to be 59%, in qualitative agreement with that predicted by a rigid radical impulsive model. Both the spin-orbit ground and excited chlorine atom angular distributions were close to isotropic. We compare the observed Cl( 2 P 1/2 )/͓Cl( 2 P 1/2 )ϩCl( 2 P 3/2 )͔ ratio produced in the electronic predissociation channel of allyl chloride with a prior study of the chlorine atom spin-orbit states produced from HCl photodissociation, concluding that angular momentum recoupling in the exit channel at long interatomic distance determines the chlorine atom spin-orbit branching.
Ultraviolet photodissociation of HCl in selected rovibrational states: Experiment and theory
The Journal of Chemical Physics, 2000
Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl (X 1 ⌺ ϩ ) molecules. Jet-cooled H 35 Cl molecules within a time-of-flight mass spectrometer were prepared by infra-red absorption in the following quantum states: vϭ1, Jϭ0 and Jϭ5; vϭ2, Jϭ0 and Jϭ11; vϭ3, Jϭ0 and Jϭ7. The excited molecules were then photodissociated at ϳ235 nm and the Cl( 2 P j ) photofragments detected using ͑2ϩ1͒ resonance enhanced multiphoton ionization. The results are presented as the fraction of total chlorine yield formed in the spin-orbit excited state, Cl( 2 P 1/2 ). The experimental measurements are compared with the theoretical predictions from a time-dependent, quantum dynamical treatment of the photodissociation dynamics of HCl (vϭ1Ϫ3, Jϭ0͒. These calculations involved wavepacket propagation using the ab initio potential energy curves and coupling elements previously reported by Alexander, Pouilly, and Duhoo ͓J. Chem. Phys. 99, 1752 ͑1993͔͒. The experimental results and theoretical predictions share a common qualitative trend, although quantitative agreement occurs only for HCl (vϭ2).
2000
Velocity distributions for the Cl( 2 P 3/2 ) and Cl( 2 P 1/2 ) photofragments produced by photolysis of Cl 2 in the region between 310 and 470 nm are measured using photofragment velocity mapping. Our results indicate that at short wavelengths the absorption spectrum is dominated by the 1 u ( 1 ⌸ u ) excited electronic state which produces two ground state chlorine atoms. The 0 u ϩ (B 3 ⌸ u ) state which produces a spin-orbit excited and a ground state chlorine atom becomes significant at 350 nm and dominates the spectrum beyond 400 nm. Analysis of the photofragment angular distributions indicates that the Cl( 2 P 3/2 ) photofragments are aligned and the magnitude of the alignment is quantitatively determined. Nonadiabatic curve crossing between the 1 u ( 1 ⌸ u ) and the 0 u ϩ (B 3 ⌸ u ) electronic states is observed and quantified below 370 nm. The measured nonadiabatic transition probability is modeled using the Landau-Zener formula and the position of the curve crossing is estimated at ϳ3 eV above the zero-point of ground electronic state of Cl 2 .
Ultraviolet photodissociation of HCl in selected rovibrational states: Experiment …
The Journal of …
Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl (X 1 ⌺ ϩ ) molecules. Jet-cooled H 35 Cl molecules within a time-of-flight mass spectrometer were prepared by infra-red absorption in the following quantum states: vϭ1, Jϭ0 and Jϭ5; vϭ2, Jϭ0 and Jϭ11; vϭ3, Jϭ0 and Jϭ7. The excited molecules were then photodissociated at ϳ235 nm and the Cl( 2 P j ) photofragments detected using ͑2ϩ1͒ resonance enhanced multiphoton ionization. The results are presented as the fraction of total chlorine yield formed in the spin-orbit excited state, Cl( 2 P 1/2 ). The experimental measurements are compared with the theoretical predictions from a time-dependent, quantum dynamical treatment of the photodissociation dynamics of HCl (vϭ1Ϫ3, Jϭ0͒. These calculations involved wavepacket propagation using the ab initio potential energy curves and coupling elements previously reported by Alexander, Pouilly, and Duhoo ͓J. Chem. Phys. 99, 1752 ͑1993͔͒. The experimental results and theoretical predictions share a common qualitative trend, although quantitative agreement occurs only for HCl (vϭ2).
The Journal of Chemical Physics, 2000
Molecular chlorine was photolyzed using circularly polarized radiation at 310 and 330 nm, and orientation moments of the chlorine-atom Cl( 2 P j ) photofragment distributions were measured by resonance enhanced multiphoton ionization using circularly polarized light with Doppler resolution. The product atoms were found to be strongly oriented in the laboratory as a result of both incoherent and coherent dissociation mechanisms, and the orientation moments contributed by each of these mechanisms have been separately measured. The experimental results can be explained by nonadiabatic transitions from the C 1 ⌸ 1u state to higher states of ⍀ϭ1 u symmetry, induced by radial derivative coupling. Ab initio calculations indicate strong Rosen-Zener-Demkov noncrossing-type radial derivative couplings between states of 1 u symmetry. The observed angular distribution ͑ parameter͒ indicates that 88% of Cl*( 2 P 1/2 ) fragments produced at 310 nm originate from a perpendicular transition to the C state. The orientation measurements suggest that 67Ϯ16% of 35 Cl*( 2 P 1/2 ) atoms dissociate via the 1 u ( 3 ⌺ 1u ϩ ) state, and 21Ϯ6% dissociate via the 1 u ( 3 ⌬ 1u ) state.
A complete quantum mechanical study of chlorine photodissociation
The Journal of Chemical Physics, 2012
A fully quantum mechanical dynamical calculation on the photodissociation of molecular chlorine is presented. The magnitudes and phases of all the relevant photofragment T-matrices have been calculated, making this study the computational equivalent of a "complete experiment," where all the possible parameters defining an experiment have been determined. The results are used to simulate cross-sections and angular momentum polarization information which may be compared with experimental data. The calculations rigorously confirm the currently accepted mechanism for the UV photodissociation of Cl 2 , in which the majority of the products exit on the C 1 1u state, with non-adiabatic couplings to the A 3 1u and several other = 1 states, and a small contribution from the B 3 0 + u state present at longer wavelengths.
The Journal of Chemical Physics, 2003
The dynamics of chlorine and hydrogen atom formation in the 193.3 nm gas-phase laser photolysis of room-temperature 1,1-dichloro-1-fluoroethane, CH 3 CFCl 2 ͑HCFC-141b͒, were studied by means of the pulsed-laser-photolysis and laser-induced fluorescence ͑LIF͒ ''pump-and-probe'' technique. Nascent ground-state Cl(2 P 3/2) and spin-orbit excited Cl*(2 P 1/2) as well as H(2 S) atom photofragments were detected under collision-free conditions by pulsed Doppler-resolved laser-induced fluorescence measurements employing narrow-band vacuum ultraviolet probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in krypton. Using HCl photolysis as a reference source of well-defined Cl(2 P 3/2), Cl*(2 P 1/2), and H atom concentrations, values for the chlorine-atom spin-orbit branching ratio ͓Cl*͔/͓Cl͔ϭ0.36Ϯ0.08, the total chlorine atom quantum yield (⌽ ClϩCl * ϭ1.01Ϯ0.14), and the H atom quantum yield (⌽ H ϭ0.04Ϯ0.01) were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the mean relative translational energy of the chlorine fragments could be determined to be E T(Cl) ϭ157Ϯ12 kJ/mol and E T(Cl *) ϭ165 Ϯ12 kJ/mol. The corresponding average values 0.56 and 0.62 of the fraction of total available energy channeled into CH 3 CFClϩCl/Cl* product translational energy were found to lie between the limiting values 0.36 and 0.85 predicted by a soft impulsive and a rigid rotor model of the CH 3 CFCl 2 →CH 3 CFClϩCl/Cl* dissociation processes, respectively. The measured total chlorine atom quantum yield along with the rather small H atom quantum yield as well as the observed energy disposal indicates that direct C-Cl bond cleavage is the most important primary fragmentation mechanism for CH 3 CFCl 2 after photoexcitation in the first absorption band.